Voltage-controlled oscillators (VCO's) designed for rapid changes in the resonant frequency have a multitude of uses in the communications and computer industries. In a VCO, the resonant frequency is determined by the capacitance of the tuning diodes. This capacitance in turn varies with the voltage impressed across the planar PN junction.
In VCO's designed for rapid changes in the resonant frequency (fast hop capability), it is important that the circuits settle at the new frequency within a short period of time--in the order of milliseconds. VCO's using tuning diodes that have oxide passivated silicon PN junctions often suffer from slow settling times because of unwanted surface charge or positively charged ions near the PN interface in the silicon oxide.
Additionally, ionizing radiation aggravates the settling time problem seen in fast hop applications. Ionizing radiation causes a buildup of trapped positive charges in the oxide near the silicon surface which adds to unwanted charge often resulting from processing steps used in the manufacture of the varactor tuning diode.
The capacitance associated with a PN junction at a given bias voltage is determined by the width of the space-charge region. As the impressed voltage is increased in the reversed-biased mode, the space-charge region widens approximately as the square root of the voltage. The capacitance of the diode varies inversely with the width of the space-charge region.
VCO's include a control loop as a part of the total circuitry for maintaining this impressed voltage. In this control loop, as the frequency of oscillation approaches the new frequency, it is sampled and measured against the selected new frequency. The difference between the sampled frequency and the selected new frequency is converted to an "error" voltage, which iteratively adjusts the voltage across the tuning diode, eventually reaching the capacitance associated with the selected new frequency of oscillation. The elapsed time the control loop circuit requires to reach convergence when changing frequency f1 to frequency f2 (or from f2 to f1), where f1 is the lower frequency, is called settling time.
The problem is that unwanted mobile positive charge, bends the depletion region of the PN junction at the surface because of the negative "image charges", slightly altering the capacitance, and its associated frequency of oscillation.
It takes a relatively long time, in the order of milliseconds, for these mobile positive charges to be "swept" away by the electric field of the expanding space-charge region. This electric field alters the density of "surface states" and the amount of bending at the surface.
In addition, the capacitance associated with each new bias voltage iteration during a fast-hop changes with time, lagging the equilibrium capacitance slightly. This also delays the control loop in the VCO from converging upon f2 or f1. The result is an excessively long settling time.
In accordance with the present invention, a varactor tuning diode for use in a voltage controlled oscillator has a metal-oxide-silicon (MOS) structure that forces the space-charge region to terminate in the oxide layer a significant distance away from the planar PN junction.
The oxide over the planar PN junction is made thin enough so that a voltage bias on the metallization immediately above the thin oxide layer causes an inversion layer in the lightly-doped N-type silicon layer immediately underneath. The design and thickness of the oxide layer is determined by the lowest voltage imposed upon the tuning diode during operation of the VCO. This causes the area under the thin oxide layer to remain in an inverted state.
Although the PN junction in this invention does terminate at the surface like any planar junction and, as such, its space-charge region at that termination will be altered by the unwanted positive charge in the oxide; these surface states will not affect the VCO settling time.
The capacitance drift, which also slows the settling time, associated with the surface states has to be charged and discharged by the signal impressed across the tuning diode. The resistance of the inversion layer, from where it terminates at the oxide layer to the PN junction, is designated to be too high to allow effective charging and discharging. Therefore, the varactor tuning effect is controlled by bulk properties of the silicon, where dielectric relaxation constants are measured in picoseconds, rather than at the silicon surface where the relaxation time is measured in milliseconds. Thus, the settling time of the varactor tuning diode of this invention is faster than the prior art which used tuning diodes without this isolating structure.
In addition, a varactor tuning diode of the present invention significantly increases the tolerance of the tuning diode to ionizing radiation. Finally, a varactor tuning diode of the present invention enables the tuning diode to behave independently of the production history of oxide cleanliness--an advantage when using a VCO in fast-hop applications.